1. Field of the Invention
The present invention relates to a resin composition for laser engraving, a relief printing plate precursor for laser engraving, a relief printing plate, and a method of producing a relief printing plate.
2. Description of the Related Art
As a method for forming a printing plate by forming a concave-convex structure on a photosensitive resin layer laminated on the surface of a support, a method of exposing a relief forming layer which has been formed using a photosensitive composition, to ultraviolet radiation through an original image film so as to selectively cure image areas, and removing uncured parts by means of a developer solution, that is, so-called “analogue plate making”, is well known.
A relief printing plate is a letterpress printing plate having a relief layer with a concave-convex structure, and such a relief layer having a concave-convex structure may be obtained by patterning a relief forming layer formed from a photosensitive composition containing, as a main component, for example, an elastomeric polymer such as synthetic rubber, a resin such as a thermoplastic resin, or a mixture of a resin and a plasticizer, to thus form a concave-convex structure. Among such relief printing plates, a printing plate having a flexible relief layer is often referred to as a flexo plate.
In the case of producing a relief printing plate by analogue plate making, since an original image film using a silver salt material is needed in general, the plate making process requires time and costs for the production of original image films. Furthermore, since chemical treatments are required in the development of original image films, and also treatments of development waste water are necessary, investigations on simpler methods of plate making, for example, methods which do not use original image films or methods which do not necessitate development treatments, are being undertaken.
In recent years, a method of making a plate having a relief forming layer by means of scanning exposure, without requiring an original image film, is being investigated. As a technique which does not require an original image film, there has been proposed a relief printing plate precursor in which a laser-sensitive type mask layer element capable of forming an image mask is provided on a relief forming layer (see, for example, Japanese Patent No. 2773847 and Japanese Patent Application Laid-Open (JP-A) No. 9-171247). The method of making such a plate precursor is referred to as a “mask CTP method”, because an image mask having the same function as the original image film is formed from the mask layer element by means of laser irradiation that is based on image data. This method does not require an original image film, but the subsequent plate making treatment involves a process of exposing the plate precursor to ultraviolet radiation through an image mask, and then removing uncured parts by development, and from the viewpoint of requiring a development treatment, the method has a room for further improvement.
As a method of plate making which does not require a development process, a so-called “direct engraving CTP method”, in which plate making is carried out by directly engraving a relief forming layer using laser, has been proposed a number of times. The direct engraving CTP method is literally a method of forming a concave-convex structure which will serve as relief, by engraving the structure with laser. This method is advantageous in that the relief shape can be freely controlled, unlike the relief formation processes using original image films. For this reason, in the case of forming images like cutout characters, it is possible to engrave the image regions deeper than other regions, or for microdot images, to carry out shouldered engraving in consideration of resistance to the printing pressure, or the like. However, in this method, since high energy is required to form a relief having a concave-convex structure which can withstand the printing pressure, on a relief forming layer having a predetermined thickness, and the speed of laser engraving is slow, the method has a problem of low productivity as compared to the methods in which image formation involves the use of a mask.
For this reason, it has been attempted to enhance the sensitivity of a relief printing plate precursor. For example, a flexographic printing plate precursor for laser engraving which includes an elastomer foam has been proposed (see JP-A No. 2002-357907). In this technology, an attempt is made to improve the engraving sensitivity by using a low density foamed material in a relief forming layer. However, due to being a foamed material having low density, the obtained printing plate has problems such as lack of strength, and seriously impaired print durability.
Japanese Patent No. 2846954, and JP-A Nos. 11-338139 and 11-170718 disclose flexographic printing plate precursors which make possible of laser engraving, or flexo plates obtained by laser engraving. According to these documents, flexo plates are obtained by incorporating a monomer as a binder into an elastomeric rubber, curing the mixture by means of a thermopolymerization mechanism or photopolymerization mechanism, and then performing laser engraving thereon.
As a problem faced by the direct engraving CTP method, the slow speed of laser engraving may be mentioned. This is because in the mask CTP method, the thickness of the mask layer element of a subject requiring abrasion is only about 1 μm to 10 μm, whereas in the direct engraving CTP method, it is necessary to engrave at least 100 μm in view of the function of directly forming a relief. Therefore, there have been several suggestions attempting to improve the laser engraving sensitivity.
For example, a flexographic printing plate precursor for laser engraving which contains an elastomer foam has been proposed (see JP-A No. 2000-318330). In this technology, an attempt is made to improve the engraving sensitivity by using a low density foamed material; however, due to being a foamed material having low density, the obtained printing plate has problems such as the lack of strength, and seriously impaired print durability.
In another example, a flexographic printing plate precursor for laser engraving which contains microspheres encapsulating a hydrocarbon-based gas has been proposed (see U.S. Patent Application Laid-Open No. 2003/180636). In this technology, an attempt is made to improve the engraving sensitivity by means of a system in which the gas inside the microspheres expands under the heat generated by laser, and disintegrates the material being engraved. However, due to being a material system containing gas bubbles, the microsphere system has a problem that the obtained printing plate is likely to lack the strength. Furthermore, since a gas has a nature of being more likely to expand under heat than solids, even though microspheres having a high thermal deformation initiation temperature are selected, volume changes due to the changes in the outside temperature are unavoidable. Therefore, it is not appropriate to use a material containing gas bubbles in the printing plates where stability in the thickness precision is required.
In still another example, a resin letterpress printing plate for laser engraving which contains a polymeric filler having a ceiling temperature of less than 600 K has been proposed (see JP-A No. 2000-168253). In this technology, an attempt is made to improve the engraving sensitivity by adding a polymeric filler having a low depolymerization temperature. However, when such a polymeric filler is used, surface irregularities are generated on the surface of the printing plate precursor, and seriously affect the printing quality.
As discussed above, a variety of technologies have been proposed in relation to a resin composition which can be suitably used in the relief forming layer of relief printing plate precursors for laser engraving, but under the current situation, a resin composition exhibiting high engraving sensitivity when submitted to laser engraving, is yet to be provided.